Month: May 2013

Inside the eye, tiny photoreceptors called cones are activated by light. Each cone is stimulated by a different color, and together, the three types of cones make up the wonderful spectrum of colors we can see.

But did you know that cone receptors in the human eye lose their color sensitivity with age? Interestingly, our subjective experience of color remains largely unchanged over the years. This ability to compensate for age-related changes in color perception rests in higher levels of the visual system – the brain – according to scientists.

The research was published May 8 in the open access journal PLOS ONEby Sophie Wuerger from the University of Liverpool, UK.

The study included 185 participants aged 18 to 75 years with normal color vision, and revealed that the appearance of color remains largely unaffected by known age-related changes in the optical media within the lens, but the ability to distinguish between small differences in shades of colors decreases with increasing age, particularly for colors on the yellow-blue axis.

The effects of age were most apparent when participants viewed shades of green in daylight- what appeared uniquely green to younger observers appeared more yellowish to older viewers. The author concludes that certain neural pathways compensate for age-related losses in the eye, so color functions remain largely constant over time.

“We found that colour vision remains fairly constant across the life span, despite the known age-related yellowing of the lens,” says Wuerger. “This suggests that the visual brain re-calibrates itself as we get older.”

As is the theme of recent blog posts, the eye simply collects data and the brain develops that data into the world we see.

How does San Francisco Giants slugger Pablo Sandoval hit a 95 mph fastball, or tennis star Venus Williams return her sister Serena’s 120 mph serves? For the first time, vision scientists at the University of California, Berkeley, have identified how our brains track fast-moving objects.

The discovery advances our understanding of how humans predict the trajectory of moving objects. Amazingly, it can take as little as one-tenth of a second for the brain to process what the eye sees.

But a lot can happen in that 100-millisecond holdup. In real time, a tennis ball moving at 120 mph would have already traveled 15 feet before the brain registers the ball’s location! If our brains couldn’t make up for this visual processing delay, we’d be constantly hit by balls, cars and everyday objects.

Thankfully, the brain “pushes” forward moving objects so we perceive them as further along in their trajectory than the eye can see, researchers said.

For the first time, we can see this sophisticated prediction mechanism at work in the human brain,” said Gerrit Maus, a postdoctoral fellow in psychology at UC Berkeley and lead author of the paper published May 8 in the journal, Neuron.

A clearer understanding of how the brain processes visual input, in this case “life in motion,” can eventually help in diagnosing and treating a myriad of disorders, including those that impair motion perception. People who cannot perceive motion cannot predict locations of objects and therefore cannot perform tasks as simple as pouring a cup of coffee or crossing a road, researchers said.

This study is also likely to have a major impact on other studies of the brain. Its findings come just as the Obama Administration initiates its push to create a Brain Activity Map Initiative, which will further pave the way for scientists to create a road map of human brain circuits, similar to what was done for the Human Genome Project.

Using functional Magnetic Resonance Imaging (fMRI) Gerrit and fellow UC Berkeley researchers Jason Fischer and David Whitney located the part of the visual cortex that makes calculations to compensate for our sluggish visual processing abilities. They saw this prediction mechanism in action, and their findings suggest that the middle temporal region of the visual cortex known as V5 is computing where moving objects are most likely to end up.

For the experiment, six volunteers had their brains scanned, via fMRI, as they viewed the “flash-drag effect,” a visual illusion in which we see brief flashes shifting in the direction of the motion.

“The brain interprets the flashes as part of the moving background, and therefore engages its prediction mechanism to compensate for processing delays,” Maus said.

The researchers found that the illusion — flashes perceived in their predicted locations against a moving background and flashes actually shown in their predicted location against a still background — created the same neural activity patterns in the V5 region of the brain. This established that V5 is where this prediction mechanism takes place, they said.

In a study published earlier this year, Maus and his fellow researchers pinpointed the V5 region of the brain as the most likely location of this motion prediction process by successfully using transcranial magnetic stimulation, a non-invasive brain stimulation technique, to interfere with neural activity in the V5 region of the brain, and disrupt this visual position-shifting mechanism.

“Now not only can we see the outcome of prediction in area V5,” Maus said. “But we can also show that it is causally involved in enabling us to see objects accurately in predicted positions.”

On a more evolutionary level, the latest findings reinforce that it is actually advantageous not to see everything exactly as it is. In fact, it’s necessary to our survival:

The image that hits the eye and then is processed by the brain is not in sync with the real world, but the brain is clever enough to compensate for that,” Maus said. “What we perceive doesn’t necessarily have that much to do with the real world, but it is what we need to know to interact with the real world.”